Effect of magnesium on the morphology of carbides and on M_7C_3→M_(23)C_6 carbides transformation in the 21-4N heat-resistant steel

摘要

This paper presents the effect of magnesium on the morphology of carbides and on M_7C_3→M_(23)C_6 (M = Cr and/or Fe) carbides transformation in the 21-4N heat-resistant steel. 21-4N heat-resistant steel was calculated by using the FactSage. The equilibrium carbide phase is M_(23)C_6 and is stable up to 1492 K in Mg-added 21-4N heat-resistant steel. The studies involve a wide range of experimental techniques to characterize the investigated steels in the as-cast and hot-rolled state, including X-ray diffractometer, confocal scanning laser microscope (CSLM), scanning electron microscope, transmission electron microscopes (TEM) and microhardness testers. In the as-cast and hot-rolled samples with different magnesium contents, M_7C_3 was detected due to nonequilibrium solidification, while the carbide types are M_(23)C_6 in the prediction from equilibrium phase diagram. Lamellar structure in the as-cast sample is composed of FCC austenite and M_(23)C_6. And blocky shape carbide M_7C_3 is wrapped by the lamellar structure M_(23)C_6